What is the 500 Rule?
To avoid star trails in your night photography, you need to learn and apply the 500 rule. In the photographs below you can see a close up example of what this means.
The photo on the left has a bit of star trailing, while the photograph on the right does not.
Take the number 500 and divide it by the focal point you will be shooting at (in mm) to find your exposure length. Note: your exposure should never be over 30 seconds.
An 11mm lens =45 Seconds. Since this is more than 30 seconds, 30 seconds is the longest you should shoot.
A 24mm lens = 21 Seconds
An 18mm lens = 28 Seconds
This is a guideline and there are many variables that can come into play. Personally, I do my best to keep my exposure under 15 seconds and use a 1.4 lens to compensate.
Further, practicing proper night focusing will ensure sharper stars and less blur.
By Chris Attrell